CA1178920A - Composite electrode for electrolytic processes - Google Patents
Composite electrode for electrolytic processesInfo
- Publication number
- CA1178920A CA1178920A CA000430587A CA430587A CA1178920A CA 1178920 A CA1178920 A CA 1178920A CA 000430587 A CA000430587 A CA 000430587A CA 430587 A CA430587 A CA 430587A CA 1178920 A CA1178920 A CA 1178920A
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- Prior art keywords
- layer
- ruthenium
- iridium
- barrier layer
- process according
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Abstract
ABSTRACT
A process is disclosed for producing a composite electrode for use in an electrolytic cell comprising a valve metal substrate and an outer surface layer comprising ruthenium dioxide. The improvement resides in the steps of providing a barrier layer comprising a platinum group metal. A
composite electrode for use in an electrolytic cell comprising a valve metal substrate and an outer surface layer comprising ruthenium dioxide, the improvement comprises providing a barrier layer comprising a platinum group metal directly on the substrate, and providing, between the barrier layer and outer surface layer, an intermediate layer comprising a metallic electroplated deposit consisting of ruthenium and iridium, said intermediate layer containing at least a small but effective amount of iridium to reduce ruthenium dissolution during use in said cell, said intermediate layer being at least partially oxidized.
A process is disclosed for producing a composite electrode for use in an electrolytic cell comprising a valve metal substrate and an outer surface layer comprising ruthenium dioxide. The improvement resides in the steps of providing a barrier layer comprising a platinum group metal. A
composite electrode for use in an electrolytic cell comprising a valve metal substrate and an outer surface layer comprising ruthenium dioxide, the improvement comprises providing a barrier layer comprising a platinum group metal directly on the substrate, and providing, between the barrier layer and outer surface layer, an intermediate layer comprising a metallic electroplated deposit consisting of ruthenium and iridium, said intermediate layer containing at least a small but effective amount of iridium to reduce ruthenium dissolution during use in said cell, said intermediate layer being at least partially oxidized.
Description
llt7~39~0 This invention relates to electrodes for use in electro-chemical processes, especially processes for electrowinning of metals. More particularly, the present invention relates to a process for producing a composite electrode for use in an electro-lytic cell.
With the increased emphasis that is present being placed on carrying out industrial processes with minimized environmental pollution, there has been greater interest in using electrochemical techniques for extracting metals from ores. One method currently being investigated is the electrowinning of metals, which involves the electrodeposition of a metal at the cathode when an external current is impressed on an electrolytic cell. An insoluble anode may be used, and the metal is recovered from an electrolyte which contains the metal as an ion in an appropriate solvent. Electro-winning can be used for recovering a metal from solutions derived, for example, from ores, refining processes, or even from metal scrap. Very high purity metals can be recovered using this technique, given appropriate electrodes, electrolytes and process conditions.
One of the major problems in the electrowinning of metals concerns the development of satisfactory anodes. They must be good conductors and resistant to chemical attack in the environ-ment in which they are used. They must be sufficiently strong to withstand normal handling in commercial use, and they must be effective for the desired reactions at the anode without interfering with the activity at the cathode. For example, when used in an insoluble anode in an electrowinning process, the anode should not affect adversely the purity of the metal deposit at the cathode and should not interfere with the deposit of the - 1 - r~
1178~320 metal at an economic current density. In fact, economics plays a major role in the choice of an electrode. Thus, factors which must be considered are the cost of the elec-trode, its durability and the power requirementS associated with its use. As a practical commercial reality cost of the anode not only includes cost of materials and cost of manu-facture but royalties or other expenses associated with the use or purchase of proprietary materials.
The electrodes of the present invention are particularly suited for use as insoluble anodes in the electrowinning of nickel. Accordingly the present elec-trodes are described below mainly in connection with such a process. However, it will be apparent to those skilled in the art that the present electrodes may also be employed for the electrowinning of other metals, e.g., copper, zinc, manganese, cobalt, cadmium, gallium, indium, and alloys thereof, e.g., nickel-cobalt alloys, and for other elec-trolysis processes, e.g., for the electrolytic production of chlorine from brines, the dissociation of water, cathodic protection (e.g., in seawater or underground) and for bat-tery electrodes.
In a nickel electrowinning process using insoluble anodes described by ~.R. Boldt in "The Winning of Nickel", pp. 362-374 (1967), the electrolyte used is a purified leach liquor, which is essentially an aqueous solution of nickel sulfate, sodium sulfate and boric acid and the anodes are made of rolled sheets of pure lead. The principal cathodic reaction is:
With the increased emphasis that is present being placed on carrying out industrial processes with minimized environmental pollution, there has been greater interest in using electrochemical techniques for extracting metals from ores. One method currently being investigated is the electrowinning of metals, which involves the electrodeposition of a metal at the cathode when an external current is impressed on an electrolytic cell. An insoluble anode may be used, and the metal is recovered from an electrolyte which contains the metal as an ion in an appropriate solvent. Electro-winning can be used for recovering a metal from solutions derived, for example, from ores, refining processes, or even from metal scrap. Very high purity metals can be recovered using this technique, given appropriate electrodes, electrolytes and process conditions.
One of the major problems in the electrowinning of metals concerns the development of satisfactory anodes. They must be good conductors and resistant to chemical attack in the environ-ment in which they are used. They must be sufficiently strong to withstand normal handling in commercial use, and they must be effective for the desired reactions at the anode without interfering with the activity at the cathode. For example, when used in an insoluble anode in an electrowinning process, the anode should not affect adversely the purity of the metal deposit at the cathode and should not interfere with the deposit of the - 1 - r~
1178~320 metal at an economic current density. In fact, economics plays a major role in the choice of an electrode. Thus, factors which must be considered are the cost of the elec-trode, its durability and the power requirementS associated with its use. As a practical commercial reality cost of the anode not only includes cost of materials and cost of manu-facture but royalties or other expenses associated with the use or purchase of proprietary materials.
The electrodes of the present invention are particularly suited for use as insoluble anodes in the electrowinning of nickel. Accordingly the present elec-trodes are described below mainly in connection with such a process. However, it will be apparent to those skilled in the art that the present electrodes may also be employed for the electrowinning of other metals, e.g., copper, zinc, manganese, cobalt, cadmium, gallium, indium, and alloys thereof, e.g., nickel-cobalt alloys, and for other elec-trolysis processes, e.g., for the electrolytic production of chlorine from brines, the dissociation of water, cathodic protection (e.g., in seawater or underground) and for bat-tery electrodes.
In a nickel electrowinning process using insoluble anodes described by ~.R. Boldt in "The Winning of Nickel", pp. 362-374 (1967), the electrolyte used is a purified leach liquor, which is essentially an aqueous solution of nickel sulfate, sodium sulfate and boric acid and the anodes are made of rolled sheets of pure lead. The principal cathodic reaction is:
2(Ni + 2e ~ Ni) The principal anodic reaction is:
2H20 ~ 2 + 4H + 4e It will be noted that oxygen is released at the anode.
-Lead and lead alloys have also been used as anodematerials for electrowinning of metals other than nickel, e,g., copper and zinc. The lead alloys are often mechani-cally stronger and more resistant to certain corrosive environments used in electrowinning processes than pure lead; their operating potential is substantially higher than that of precious metal coated titanium anodes, and there is the ever present possibility of cathode lead contamination, because at open circuit lead dissolves and is then available in solution for deposition at the cathode. Thus, lead has not been an entirely satisfactory anode material.
In fact, very few materials may be used effectively as anodes, especially in oxygen producing environments, because of the severe conditions. Graphite has b~en used -and its limitations are well known. In recent years there has been considerable interest in replacing graphite elec-trodes used in the electrolytic production of chlorine from brines with platinum group metal-coated anodes. In general anodes of this type are composed of a valve metal substrate having a coating containing at least one platinum group metal or platinum group metal oxide. The platinum group metal oxides have aroused attention because they are less corrosive than the elemental metals in the chloride and because there is reduced tendency to shorting in cells like the mercury cells, In particular favor recently are anode coatings composed of a platinum group metal oxide and a base metal oxide. Such coatings have been characterized by terms such as mixed crystals, solid solutions, ceramic semi-con-ductors and so on. It is reported that anodes of this 11789~C) type are now in use for the commercial production of chlorine.
Offsetting their high cost are their low power requirements and durability. Examples of the many issued patents in the field are: U.S. Patent Nos. 3,491,014, 3,616,445, 3,711,3~5,
2H20 ~ 2 + 4H + 4e It will be noted that oxygen is released at the anode.
-Lead and lead alloys have also been used as anodematerials for electrowinning of metals other than nickel, e,g., copper and zinc. The lead alloys are often mechani-cally stronger and more resistant to certain corrosive environments used in electrowinning processes than pure lead; their operating potential is substantially higher than that of precious metal coated titanium anodes, and there is the ever present possibility of cathode lead contamination, because at open circuit lead dissolves and is then available in solution for deposition at the cathode. Thus, lead has not been an entirely satisfactory anode material.
In fact, very few materials may be used effectively as anodes, especially in oxygen producing environments, because of the severe conditions. Graphite has b~en used -and its limitations are well known. In recent years there has been considerable interest in replacing graphite elec-trodes used in the electrolytic production of chlorine from brines with platinum group metal-coated anodes. In general anodes of this type are composed of a valve metal substrate having a coating containing at least one platinum group metal or platinum group metal oxide. The platinum group metal oxides have aroused attention because they are less corrosive than the elemental metals in the chloride and because there is reduced tendency to shorting in cells like the mercury cells, In particular favor recently are anode coatings composed of a platinum group metal oxide and a base metal oxide. Such coatings have been characterized by terms such as mixed crystals, solid solutions, ceramic semi-con-ductors and so on. It is reported that anodes of this 11789~C) type are now in use for the commercial production of chlorine.
Offsetting their high cost are their low power requirements and durability. Examples of the many issued patents in the field are: U.S. Patent Nos. 3,491,014, 3,616,445, 3,711,3~5,
3,732,157, 3,751,296, 3,770,613, 3,775,284, 3,778,307, 3,810,770, 3,840,443, 3,846,273, 3,853,73~, 4,003,~17,
4,070,504. A review of the patents will show that several of the coatings described may contain Ru02 or Ru02 and IrO2 and/or Ir, as well as a valve metal oxide such as TiO2.
The platinum group metals do not all exhibit the same properties when used in electrolytic cells. Their behavior will vary with electrolytic conditions and the reactions which occur. It has been found, for example, that anodes having an outer coating containing oxides cf a platinum group metal and a valve metal, e.g. RuOz and TiO2, which are presently in favor for the production of chlorine, have short life in electrowinning applications where oxygen is produced at the anode. One major problem is that the electrode is passivated, and according to one theory the passivation is caused by penetration of oxygen through the outer coating into the conductive substrate, e.g., a valve metal. Electrodes with intermediate coatings between the active surface coating and the substrate conductors have been proposed. Examples of such electrodes can be found in U.S. Patent Nos. 3,616,302, 3,775,284 and 4,028,215. ~one of the proposed electrodes are entirely satisfactory.
A review of the patents listed previously will show that many techniques have been listed for preparing _ 117E~9Z~
platinum group metal-containing coatings. Despite the convenience of applying coatings by electroplating, the emphasis appears to be on the "paint" application of a platinum group metal compound which will react with oxygen when heated in air to form an oxide, e.g., RuCl3 is con-verted to Ru02 when he~ted in air at a temperature of about 200C to about 700C. This impression is borne out bv L.D.
Burke et al in an article entitled "The Oxygen Electrode" in J.C.S. Faraday I Vol. 73 (11) 1669-1849 (1977), which indicates that RuO2-coated electrodes are usually prepared by heating RuCl3-painted titanium in air for several hours.
The article also records the investigation of the possibility of preparing Ru02 electrodes by thermal oxidation of electro-deposited ruthenium, and the finding that the electrode-posited electrode coatings were unsatisfactory from the point of oxygen potential and corrosion, the corrosion being evidenced by the appearance of a yellow color in the solution.
Elsewhere it has been reported that from the Pourbaix diagram a likely product of the dissolution of ruthenium in acidic solution is the yellow volatile tetroxide, viz. ~U04.
It has now been found that electrodes prepared with an electrodeposited ruthenium-iridium intermediate coating, which has been at least partially oxidized and which has a non-electrolytically-deposited ruthenium dioxide layer at the surface are very effective oxygen electrodes having low oxygen potentials and being durable in acid environments.
f~:i'7~9~0 It is an object of the present invention to provide an electrode material which can be used as an insoluble anode in electrolytic processes, particularly for the electrowinning of metals such as nickel, copper and zinc. It is another object to provide an electrode material which has long life and low power requirements when used as an anode in an electrolytic cell. Another object is to provide an electrode material which has corrosion resistance when used as an anode in an aqueous acid environment at the current densities and temperatures of use. Still another object is to pro-vide an electrode which is useful as an insoluble anode for the electro-winning of nickel.
These and other objects of the present invention will become apparent to those skilled in the art from the description and examples set orth below.
According to the present invention a process is provided for pro-ducing a composite electrode material which is especially useful as an in-soluble anode for electrowinning of metals, particularly nickel, where oxygen is evolved at the anode and high acid concentrations and elevated tempera-tures are used.
SUMMARY OF THE INVENTION
The present invention may generally be defined as an improved process for producing a composite electrode for use in an electrolytic cell comprising a valve metal substrate and an outer surface layer comprising ruthenium dioxide. The improvement comprises providing a barrier layer comprising a platinum group metal directly on the substrate, and providing between the barrier layer and outer surface layer, an intermediate layer comprising a metallic electroplated deposit consisting of ruthenium and iridium, said intermediate layer containing at least a small but effective amount of iridium to reduce ruthenium dissolution during use in said cell, said intermediate layer being at least partially oxidi~ed.
~i78920 The ruthenium-iridium electrodeposit may also be referred to as an alloy. "Alloy" is intended to include a mixture of very fine particles of ruthenium and iridium which has a metallic appearance. The particles may be mixed crystals or in solid solu-tion, the microscopic character of the deposited films being difficult to determine because the films are very thin.
DESCRIPTION OF PREFERRED EMBODIMENTS
-A principal feature of present invention resides in the particular combination of composition and methods of deposit-ing of the layers in the multilayer coating. The coating, as indicated previously, is on an electroconductive substrate.
The substrate, which must be electroconductive, shouldbe of a material which will be resistant to the environment in which it is used. The substrate may be, for example, a valve metal or graphite. The term "valve metals" is used in the usual sense as applied to electrode materials.
1:~78920 They are high melting, corrosion resistant, electrically conductive metals which passivate, i.e., form protective films in certain electrolytes. Examples of valve metals are titanium, tantalum, niobium, zirconium, hafnium, molybdenum, tungsten, aluminum, and alloys thereof. Titanium is a pre-ferred substrate material because of its electrical and chemical properties, its availability, and, its cost relative to other materials with comparable properties. The configu-ration of the substrate is not material to this invention.
It is well known to use electrodes in many shapes and sizes, e.g., as sheet, mesh, expanded metal, tubes, rods, etc. The titanium may be, for example, a sheath on a more conductive metal such as copper, iron, steel, or aluminum, or combina-tions thereof.
The valve metal substrate is treated to clean, and preferably to roughen the surface before any coating is applied. Cleaning includes, for example, removal of grease and dirt and also removal of any oxide skin that may have formed on the valve metal. The usual techniques may be used to roughen the surface of the valve metal, e.g., by etching or grit blasting. A particularly suitable technique is to grit blast using silica sand.
The barrier layer deposited on the substrate improves the durability of the electrode. It is believed to serve as an oxygen diffusion barrier for the substrate and/or to behave as a current carrying layer and/or to serve as a proper support layer. By proper support layer is meant that it improves the quality and adherence of the electro-deposited layer. In any event a principal function of the 1178~2~) barrier layer is to preserve the current carrying capacity of the electrode in the presence of released oxygen. The barrier layer composition is, advantageously, selected from the group consisting of platinum group metals, gold, alloys, mixtures, intermetallics, oxides thereof. It may also be a silicide, nitride, and carbide of one of the components of the substrate material. Preferably the barrier layer con-tains at least one of the platinum group metals palladium, platinum, iridium and rhodium. Palladium and iridium are preferred because they are effective in preserving the current carrying capacity of the electrodes, possibly as barriers to 2 transport, without any special treatment.
Platinum is effective but requires an additional oxidizing treatment, e.g. ~y soaking in an oxidizing medium such as in concentrated ~NO3 or O.lN KMnO4. The use of rhodium is not recommended because of its high cost.
It has also been found that silicides, nitrides and carbides of at least one component of the valve metal substrate are suitable as barrier layers. Standard tech-niques may be used to deposit such coatings on the sub-strate. These coatings are orders of magnitude greater in thickness than the platinum group metal barrier layers. For example, a nitride coating may be about 2~ thick and a silicide layer may be about 250~ thick.
In a preferred embodiment the electrode contains a palladium- or iridium-containing layer adjacent to the valve metal. The palladium layer, which serves as a barrier layer on the substrate, also promotes adherence of the ruthenium-11'78~3~0 iridium electrodeposited layer to the substrate. The palladium or iridium can be deposited in any manner, e.g., by chemical or thermal decomposition from a solution or slurry deposited on the substrate, or by electroplating, electrophoresis, etc. Electroplating is preferred because it is convenient, inexpensive, rapid, neither labor nor time intensive compared to thermal decomposition, and it is easily controlled compared to, e.g., electrophoresis or chemical or vapor deposition. The palladium layer is at least about 0.05~m in thickness. The optimum thickness is about 0.2~m. Generally, what is sought is sufficient metal to coat the substrate substantially completely. It has been found, for example that a palladium deposit of 0.25 mg/cm2 is a sufficient deposit to coat completely a sanu~iasted or otherwise roughened surface of the substrate. Iridium is more difficult to plate than palladium and it is more ex-pensive. However, a flash coating of iridium serves as an effective barrier.
Examples of known palladium electroplating baths are:
BATH I BATH II
Pd as: Pd as:
PdCl2.H20 . . 5 to 50 g/l Pd(NH3)2C1z . . 8- 16 g/l NH4Cl . . . . 20 -50 g/l NH4Cl . . . . . 60-200 g/l HCl . . . . . to maintain pH pH . . . . . . 8-9.5 pH . . 0.1-0.5 Temp. . . . . . 25-35C
Temp . . 35-50C Current Current Density . . . 10 mA/cm2 Density . . 5-10 mA/cm2 117892~
For an iridium barrier layer, the bath described in U.S. Patent No. 3,693,219 may be used.
The intermediate layer between the flash coating of palladium and the outer ruthenium-dioxide coating con-sists essentially of ruthenium and iridium which has been deposited by an electroplating technique.
While ruthenium-iridium co-deposits can be formed by a number of techniques, it is particularly advantageous for the coating to be electroplated in that a metallic coating of suitable thic]cness can be deposited in one operation, a layer of uniform composition can be formed, and the deposit can be formed rapidly, in a manner which is neither time nor labor intensive compared to chemical or thermal decomposition techni~ues.
In accordance with the present invention, the ruthenium-iridium layer is deposited in the metallic state by an electroplating technique. Preferably the layer is co-deposited although it is possible to deposit layers separately, e.g., using a ruthenium plating bath described in U.S. Patent No. 3,576,724 and an iridium plating bath described in U.S. Patent No. 3,693,219, and diffuse them thermally. While this invention is not confined to any particular electroplating method for producing thc ]ayer, an especially suitable method and bath for forming the layer can be found in Canadian Application Serial No. 330,105 r filed June 19, 1979, co-pending herewith.
As noted above, electroplated ruthenium per se '~ 11'7~39Z~
will corrode ~apidly at the anode at potentials for oxygen evolution, passing into the acid solution i.n the octavalent state at potentials greater than about l.l V (vs. SCE).
This is both costly - in the loss of expensive precious metals - and a hazard in that there is a potential for vaporization of Ru04. It has been found that iridium ad-dition in the electrodeposited coating suppresses the dissolution of rutheniurn. l'he level of iridium addition which is effective depends on the conditions under which the anode is used. Very small additions of iridium have a marked effect in suppressing the ruthenium dissolution. For example, in an accelerated life test in sulfuric acid at a current density of 500 mA/cm2 and ambient temperature, roughly l weight % iridium addition increased the anode life from l hour (without iridium addition) to at least ll hours, and even as high as 95 hours, and similarly 2 weight %
iridium further increased the anode life. The iridium addition is typically in the range of about l~ up to about 36%, For electrowinning of nickel, e.g. at current densities of the order of 30 to 50 mA/cm2 and temperatures of about 55 to 80C, very small additions of iridium are effective. In an advantageous embodiment of the invention for use at current densities up to about 50 mA/cm2, the level of iridium in the electrodeposited layer is at least about and preferably greater than about 1~, e.g. about 2~
or 4%. For example, in such anodes having a further outer layer of non-electroplated RuO2, there is no observable dissolution of ruthenium with an iridium level of about 4 weight %. When used for current densities greater than about 50 mA/cm2, the iridium level is preferably at least about 2~.
~.lt~892~
Without the RuOz outer layer a greater amount of iridium is required than 4%, e.g., 7%, to prevent ruthenium disso-lution. Even at the higher levels of iridium, e.g. 7%, the metallic electrodeposited layer must be subjected to an oxidizing treatment to oxidize the surface at least partially.
Where more severe electrolysis conditions are used, a greater amount of iridium may be necessary to suppress ruthenium dissolution.
It was noted that even with the anodes where the iridium content was not sufficiently high for ruthenium dissolution to occur initially, in use anodically an oxide coating builds up which eventually protects the coating and prevents further dissolution of the ruthenium. However, to avoid the initial dissolution and to avoid the hazard of Ru04 formation, a ruthenium dioxide-containing coating -formed by a non-electrolytic treatment - is provided on the surface of the electrode.
Before depositing a further layer on the electro-plated ruthenium-iridium coating, however, the ruthenium-iridium alloy layer is treated in air to at least partially oxidize the surface. By this is meant the surface can be partially oxidized or essentially fully oxidized or the layer can be partially or essentially fully oxidized to any depth in the layer. Surface oxidation of the intermediate layer can be carried out at a temperature about 400C to about 900C in an atmosphere which is oxidizing to the deposit. Air is preferred.
In a preferred embodiment, heat treatment of the 1178~21~
intermediate layer is carried out at about 400C to about 700C, e.g., about 593C for about 5 to about 60 minutes, e.~., about 15 minutes. Advantageously the ruthenium-iridium layer has a thickness of about O.l~m to about 4 or
The platinum group metals do not all exhibit the same properties when used in electrolytic cells. Their behavior will vary with electrolytic conditions and the reactions which occur. It has been found, for example, that anodes having an outer coating containing oxides cf a platinum group metal and a valve metal, e.g. RuOz and TiO2, which are presently in favor for the production of chlorine, have short life in electrowinning applications where oxygen is produced at the anode. One major problem is that the electrode is passivated, and according to one theory the passivation is caused by penetration of oxygen through the outer coating into the conductive substrate, e.g., a valve metal. Electrodes with intermediate coatings between the active surface coating and the substrate conductors have been proposed. Examples of such electrodes can be found in U.S. Patent Nos. 3,616,302, 3,775,284 and 4,028,215. ~one of the proposed electrodes are entirely satisfactory.
A review of the patents listed previously will show that many techniques have been listed for preparing _ 117E~9Z~
platinum group metal-containing coatings. Despite the convenience of applying coatings by electroplating, the emphasis appears to be on the "paint" application of a platinum group metal compound which will react with oxygen when heated in air to form an oxide, e.g., RuCl3 is con-verted to Ru02 when he~ted in air at a temperature of about 200C to about 700C. This impression is borne out bv L.D.
Burke et al in an article entitled "The Oxygen Electrode" in J.C.S. Faraday I Vol. 73 (11) 1669-1849 (1977), which indicates that RuO2-coated electrodes are usually prepared by heating RuCl3-painted titanium in air for several hours.
The article also records the investigation of the possibility of preparing Ru02 electrodes by thermal oxidation of electro-deposited ruthenium, and the finding that the electrode-posited electrode coatings were unsatisfactory from the point of oxygen potential and corrosion, the corrosion being evidenced by the appearance of a yellow color in the solution.
Elsewhere it has been reported that from the Pourbaix diagram a likely product of the dissolution of ruthenium in acidic solution is the yellow volatile tetroxide, viz. ~U04.
It has now been found that electrodes prepared with an electrodeposited ruthenium-iridium intermediate coating, which has been at least partially oxidized and which has a non-electrolytically-deposited ruthenium dioxide layer at the surface are very effective oxygen electrodes having low oxygen potentials and being durable in acid environments.
f~:i'7~9~0 It is an object of the present invention to provide an electrode material which can be used as an insoluble anode in electrolytic processes, particularly for the electrowinning of metals such as nickel, copper and zinc. It is another object to provide an electrode material which has long life and low power requirements when used as an anode in an electrolytic cell. Another object is to provide an electrode material which has corrosion resistance when used as an anode in an aqueous acid environment at the current densities and temperatures of use. Still another object is to pro-vide an electrode which is useful as an insoluble anode for the electro-winning of nickel.
These and other objects of the present invention will become apparent to those skilled in the art from the description and examples set orth below.
According to the present invention a process is provided for pro-ducing a composite electrode material which is especially useful as an in-soluble anode for electrowinning of metals, particularly nickel, where oxygen is evolved at the anode and high acid concentrations and elevated tempera-tures are used.
SUMMARY OF THE INVENTION
The present invention may generally be defined as an improved process for producing a composite electrode for use in an electrolytic cell comprising a valve metal substrate and an outer surface layer comprising ruthenium dioxide. The improvement comprises providing a barrier layer comprising a platinum group metal directly on the substrate, and providing between the barrier layer and outer surface layer, an intermediate layer comprising a metallic electroplated deposit consisting of ruthenium and iridium, said intermediate layer containing at least a small but effective amount of iridium to reduce ruthenium dissolution during use in said cell, said intermediate layer being at least partially oxidi~ed.
~i78920 The ruthenium-iridium electrodeposit may also be referred to as an alloy. "Alloy" is intended to include a mixture of very fine particles of ruthenium and iridium which has a metallic appearance. The particles may be mixed crystals or in solid solu-tion, the microscopic character of the deposited films being difficult to determine because the films are very thin.
DESCRIPTION OF PREFERRED EMBODIMENTS
-A principal feature of present invention resides in the particular combination of composition and methods of deposit-ing of the layers in the multilayer coating. The coating, as indicated previously, is on an electroconductive substrate.
The substrate, which must be electroconductive, shouldbe of a material which will be resistant to the environment in which it is used. The substrate may be, for example, a valve metal or graphite. The term "valve metals" is used in the usual sense as applied to electrode materials.
1:~78920 They are high melting, corrosion resistant, electrically conductive metals which passivate, i.e., form protective films in certain electrolytes. Examples of valve metals are titanium, tantalum, niobium, zirconium, hafnium, molybdenum, tungsten, aluminum, and alloys thereof. Titanium is a pre-ferred substrate material because of its electrical and chemical properties, its availability, and, its cost relative to other materials with comparable properties. The configu-ration of the substrate is not material to this invention.
It is well known to use electrodes in many shapes and sizes, e.g., as sheet, mesh, expanded metal, tubes, rods, etc. The titanium may be, for example, a sheath on a more conductive metal such as copper, iron, steel, or aluminum, or combina-tions thereof.
The valve metal substrate is treated to clean, and preferably to roughen the surface before any coating is applied. Cleaning includes, for example, removal of grease and dirt and also removal of any oxide skin that may have formed on the valve metal. The usual techniques may be used to roughen the surface of the valve metal, e.g., by etching or grit blasting. A particularly suitable technique is to grit blast using silica sand.
The barrier layer deposited on the substrate improves the durability of the electrode. It is believed to serve as an oxygen diffusion barrier for the substrate and/or to behave as a current carrying layer and/or to serve as a proper support layer. By proper support layer is meant that it improves the quality and adherence of the electro-deposited layer. In any event a principal function of the 1178~2~) barrier layer is to preserve the current carrying capacity of the electrode in the presence of released oxygen. The barrier layer composition is, advantageously, selected from the group consisting of platinum group metals, gold, alloys, mixtures, intermetallics, oxides thereof. It may also be a silicide, nitride, and carbide of one of the components of the substrate material. Preferably the barrier layer con-tains at least one of the platinum group metals palladium, platinum, iridium and rhodium. Palladium and iridium are preferred because they are effective in preserving the current carrying capacity of the electrodes, possibly as barriers to 2 transport, without any special treatment.
Platinum is effective but requires an additional oxidizing treatment, e.g. ~y soaking in an oxidizing medium such as in concentrated ~NO3 or O.lN KMnO4. The use of rhodium is not recommended because of its high cost.
It has also been found that silicides, nitrides and carbides of at least one component of the valve metal substrate are suitable as barrier layers. Standard tech-niques may be used to deposit such coatings on the sub-strate. These coatings are orders of magnitude greater in thickness than the platinum group metal barrier layers. For example, a nitride coating may be about 2~ thick and a silicide layer may be about 250~ thick.
In a preferred embodiment the electrode contains a palladium- or iridium-containing layer adjacent to the valve metal. The palladium layer, which serves as a barrier layer on the substrate, also promotes adherence of the ruthenium-11'78~3~0 iridium electrodeposited layer to the substrate. The palladium or iridium can be deposited in any manner, e.g., by chemical or thermal decomposition from a solution or slurry deposited on the substrate, or by electroplating, electrophoresis, etc. Electroplating is preferred because it is convenient, inexpensive, rapid, neither labor nor time intensive compared to thermal decomposition, and it is easily controlled compared to, e.g., electrophoresis or chemical or vapor deposition. The palladium layer is at least about 0.05~m in thickness. The optimum thickness is about 0.2~m. Generally, what is sought is sufficient metal to coat the substrate substantially completely. It has been found, for example that a palladium deposit of 0.25 mg/cm2 is a sufficient deposit to coat completely a sanu~iasted or otherwise roughened surface of the substrate. Iridium is more difficult to plate than palladium and it is more ex-pensive. However, a flash coating of iridium serves as an effective barrier.
Examples of known palladium electroplating baths are:
BATH I BATH II
Pd as: Pd as:
PdCl2.H20 . . 5 to 50 g/l Pd(NH3)2C1z . . 8- 16 g/l NH4Cl . . . . 20 -50 g/l NH4Cl . . . . . 60-200 g/l HCl . . . . . to maintain pH pH . . . . . . 8-9.5 pH . . 0.1-0.5 Temp. . . . . . 25-35C
Temp . . 35-50C Current Current Density . . . 10 mA/cm2 Density . . 5-10 mA/cm2 117892~
For an iridium barrier layer, the bath described in U.S. Patent No. 3,693,219 may be used.
The intermediate layer between the flash coating of palladium and the outer ruthenium-dioxide coating con-sists essentially of ruthenium and iridium which has been deposited by an electroplating technique.
While ruthenium-iridium co-deposits can be formed by a number of techniques, it is particularly advantageous for the coating to be electroplated in that a metallic coating of suitable thic]cness can be deposited in one operation, a layer of uniform composition can be formed, and the deposit can be formed rapidly, in a manner which is neither time nor labor intensive compared to chemical or thermal decomposition techni~ues.
In accordance with the present invention, the ruthenium-iridium layer is deposited in the metallic state by an electroplating technique. Preferably the layer is co-deposited although it is possible to deposit layers separately, e.g., using a ruthenium plating bath described in U.S. Patent No. 3,576,724 and an iridium plating bath described in U.S. Patent No. 3,693,219, and diffuse them thermally. While this invention is not confined to any particular electroplating method for producing thc ]ayer, an especially suitable method and bath for forming the layer can be found in Canadian Application Serial No. 330,105 r filed June 19, 1979, co-pending herewith.
As noted above, electroplated ruthenium per se '~ 11'7~39Z~
will corrode ~apidly at the anode at potentials for oxygen evolution, passing into the acid solution i.n the octavalent state at potentials greater than about l.l V (vs. SCE).
This is both costly - in the loss of expensive precious metals - and a hazard in that there is a potential for vaporization of Ru04. It has been found that iridium ad-dition in the electrodeposited coating suppresses the dissolution of rutheniurn. l'he level of iridium addition which is effective depends on the conditions under which the anode is used. Very small additions of iridium have a marked effect in suppressing the ruthenium dissolution. For example, in an accelerated life test in sulfuric acid at a current density of 500 mA/cm2 and ambient temperature, roughly l weight % iridium addition increased the anode life from l hour (without iridium addition) to at least ll hours, and even as high as 95 hours, and similarly 2 weight %
iridium further increased the anode life. The iridium addition is typically in the range of about l~ up to about 36%, For electrowinning of nickel, e.g. at current densities of the order of 30 to 50 mA/cm2 and temperatures of about 55 to 80C, very small additions of iridium are effective. In an advantageous embodiment of the invention for use at current densities up to about 50 mA/cm2, the level of iridium in the electrodeposited layer is at least about and preferably greater than about 1~, e.g. about 2~
or 4%. For example, in such anodes having a further outer layer of non-electroplated RuO2, there is no observable dissolution of ruthenium with an iridium level of about 4 weight %. When used for current densities greater than about 50 mA/cm2, the iridium level is preferably at least about 2~.
~.lt~892~
Without the RuOz outer layer a greater amount of iridium is required than 4%, e.g., 7%, to prevent ruthenium disso-lution. Even at the higher levels of iridium, e.g. 7%, the metallic electrodeposited layer must be subjected to an oxidizing treatment to oxidize the surface at least partially.
Where more severe electrolysis conditions are used, a greater amount of iridium may be necessary to suppress ruthenium dissolution.
It was noted that even with the anodes where the iridium content was not sufficiently high for ruthenium dissolution to occur initially, in use anodically an oxide coating builds up which eventually protects the coating and prevents further dissolution of the ruthenium. However, to avoid the initial dissolution and to avoid the hazard of Ru04 formation, a ruthenium dioxide-containing coating -formed by a non-electrolytic treatment - is provided on the surface of the electrode.
Before depositing a further layer on the electro-plated ruthenium-iridium coating, however, the ruthenium-iridium alloy layer is treated in air to at least partially oxidize the surface. By this is meant the surface can be partially oxidized or essentially fully oxidized or the layer can be partially or essentially fully oxidized to any depth in the layer. Surface oxidation of the intermediate layer can be carried out at a temperature about 400C to about 900C in an atmosphere which is oxidizing to the deposit. Air is preferred.
In a preferred embodiment, heat treatment of the 1178~21~
intermediate layer is carried out at about 400C to about 700C, e.g., about 593C for about 5 to about 60 minutes, e.~., about 15 minutes. Advantageously the ruthenium-iridium layer has a thickness of about O.l~m to about 4 or
5~m, preferably 0.5~m to about 2~m, e.g., about l~m. The surface oxidation need only be carried out to provide an observable color change of metallic to violet. This is an evidence of surface oxidation. It is known that various oxides will develop at least at the surface of ruthenium and iridium when subjected to such oxidation treatment. The ruthenium-iridium electrodeposited layer, which is believed to be an alloy, clearly oxidizes at least at the surface. A
predominant phase present is Ru02, which may be in solid so-lution with other oxides which develop at the surface.
In view of the dependence on the conditions of use, the electrode can be designed with the appropriate amount of iridium. For reasons of cost, consistent with electrode life, it is preferable to keep the iridium level as low as possible.
The surface layer in a preferred anode of this invention contains as an essential component ruthenium dioxide which has been developed from a non-electrolytically deposited source. This, as noted above, is to ensure that even initially there is no loss of ruthenium anodically in use. Ruthenium dioxide is known to have a low oxygen over-potential, and its presence at the surface as an additional layer will also optimize the effectiveness of the material as an oxygen electrode. This in turn will enable the use of li7892~
the electrode at a sufficiently low potential to minimize the possibility of initial dissolution of ruthenium. Other non-electrolytically active components may be present, e.g.
for adherence, e.g., an oxide of substrate components such as TiO2, Ta2O5 and the like. In a preferred embodiment of the invention the outer surface layer contains at least abou-t 8090 RuO2. In the embodiment in which a non-active component is present the outer surface layer contains about 80% to about 99% ruthenium dioxide and about 1% to about 20%
of the non-active component, e.g., titanium dioxide. Suit-able outer layers may contain ~or example, 80% RuO2-20~
TiO2, 85% RuO2-15~ TiO2, 90% RuO2-10% TiO2, 80% RuO2-10%
TiO2-10~o Ta2O5. It is believed, however, that the require-ment for a non-active component such as a valve metal oxide is less critical and may even be eliminated in the present electrodes. The reason for this is -that the thickness requirements of the outer (non-electrolytic) RuO2 deposit are not as critical in the present electrodes as in conventional electrodes made entirely of a paint-type deposit. Con-ventional paint-type electrodes require a thickness build-up in sequential deposits that have been reported to be as high as 8 coatings and higher with firing steps intermit-tently in the build-up. Since the RuO2 (non-electrolytically deposited) layer can be thir,ner in the present electrodes, with no more than, for example, 1 or 2 coatings, the re-quirement for additional binders is lowered. Indeed durable anodes have been made using as the outer surface layer and a Ru-Ir layer, a RuO2 developed from paints without any ad-ditional oxide component. Where resinates, or the li~e are .
:~ 1'7~9;~0 used, some oxides may be derived from the usual commercial formulations, but such paint formulations can be applied without any additional oxides added.
Any non-electrolytic technique can be used for prGducing the ruthenium dioxide containing outer surface layer. Many methods are known, ~or example, for developing ruthenium dioxide coatings from aqueous or organic vehicles containing ruthenium values. For example, the ruthenium may be present as a compound such as a halide or resinate, which oxidizes to ruthenium dioxide when subjected to a heat treatment in an oxidizing atmosphere. Several methods for developing ruthenium dioxide surface coatings from non-electroplated coatings are described in the patents cited previously. In one method a ruthenium chloride in solution is applied as a paint and the coating of ruthenium dioxide is formed by dechlorination and oxidation of the ruthenium chloride. For example, a solution of RuCl3.3H20 in a suit-able carrier may be applied on a previously coated and treated composite by brushing, spraying or dipping. A
sufficient number of coats are applied to provide a ru-thenium content of at least about 0.1 mg/cm2 of electrode surface area. The coatings may be fired individually or each may be allowed to dry and the final coating fired.
Firing is carried out, e.g., in air at a temperature of about 315C to about 455C, e.g., about 315C to about 455C
for about 15 to about 60 minutes. Titanium or other non-active components may be co-deposited with the ruthenium using conventional techniques. Typically the initial loading (i.e. prior to build-up in use) of the RuO2-containing outer layer is at least about 0.1 mg/cm2. Preferably, the initial loading is about 0.3 to about 1 mg/cm2 in thickness. Since there is usually a build-up of RuO2 during use in the cell, the initial thickness of RuO2 is to ensure that preci~us metals of the intermediate layer do not dissolve before the proper build-up of RuO2 can occur and to ensure a low oxygen overpotential in the cell. In this way precious ~etal loss is minimized.
As indicated above, in a preferred embodiment of the invention the composite electrode is used as an insoluble anode for the electrowinning of nickel. While it is not the intention to confine the use of the electrodes to any one process, one contemplated use of the present electrode is in nickel electrowinning processes. For example, nickel electrowinning processes are known which use electrolytes containing about 40 to lO0 g/l nickel, 50 to lO0 g/l sodium sulfate and ùp to 40 g/l boric acid in sulfuric acid to maintain a pH ~n the range of about 0 to 5.5. In one such electrowinning process the anode is bagged, and the anolyte is a sulfate solution containing about 40 to 70 g/l nickel (as nickel sulfate), 40 g/l sulfuric acid, 100 g/l sodium sulfate, 40 g/l boric acid, and the anolyte is at a pH of about 0.
Electrowinning is carried out advantageously at a temperature of about 50 to 70C and at an anode current denslty of about 30-50 milliamps per square centimeter (mA/cm2).
The following examples are intended to give those skilled in the art a better appreciation of the invention. In all the tests, anode potentials are measured in volts vs. a saturated calomel electrode (SCE) and H/T is an abbreviation 11'~89ZO
to denote the conditioning of the layer of a composite sample, viz. the temperature, time and atmosphere. Load-ings, e.g. of precious metals or their oxides, alloys, etc., in various layers are given as nominal values.
EXAMPLE I
This example illustrates the preparation of typical electrodes of the present invention, in which the barrier layer is palladium, and the activity of such electrodes when used as anodes for the electrowinning of nickel.
Several multilayer samples are prepared on a titanium substrate material as follows:
Surface roughened titanium sheet is cleaned and plated with a thin coating of a precious metal as a barrier layer.
To roughen and clean the titanium it is sandblasted with SiOz-sand, brushed with pumice, rinsed, cathodically cleaned in 0.5 M Na2CO3 to remove dirt and the remaining pumice particles then rinsed and dried. Thereafter, the cleaned substrate is plated with a thin deposit of palladium, the amount varying from about 0.1 to about 0.6~m, using known electroplàting baths. In some of the samples the palladium deposit i5 subjected to special treatment. For example, the palladium coated-titanium in some samples are subjected to a temperature of 593C for 1 hour in an atmosphere of 5% ~2-Bal N2. It was found during the course of investigating the materials that such treatment of the palladium layer could be eliminated without noticeable harmful effects in the electrode life or performance.
A ruthenium-iridium intermediate, e.g., of about .
1/2 to about 4~m thickness, is plated on the palladium layer from a sulfamate bath to give a deposit containing about 4~
iridium and the balance ruthenium. The bath, which is dis-closed in the co-pending application referred to above, is maintained at a pH of 0.9 and a temperature of 57C and operated at a current density of 20 mA/cm2. The ruthenium-iridium deposit is treated in air at a temperature of about 500 to 600C for about 10 to 20 minutes to oxidize the surface.
The surface Ru02 layer is applied to each sample by painting the composite with 2 coats of a solution of RuCl3-3H20 in n-butanol. After each application the elec-trode is dried under a heat lamp (about 65-93C) to obtain a ru~henium chloride loading of about 1 mg/cm2, and then the composite is heat treated in air for 60 minutes at about 450C to about 600C in order to convert the chloride to the dioxide of ruthenium.
A uniform, blue-black coating results which is adherent when finger rubbed, but not completely adherent when subjected to a tape test. The tape test involves firmly applying a strip of tape to the coating and rapidly stripping the tape off. The tape is then examined to see whether any of the coating has been pulled off from the substrate.
The samples are tested as anodes under conditions which simulate the anolyte in a bagged-anode nickel electro-winning, viz. an aqueous electrolyte composed of 70 g/l nickel (as nickel sulfate), 40 g/l sulfuric acid, 100 g/l ~17892(~
sodium sulfate, and 10 g/l boric acid. The bath is main-tained at a temperature of 70C, a pH of O to 0.5, and an anode current density of 30 mA/cm2. The tests are arbi-trarily terminated when the anode potential reaches 2 volts (vs. SCE).
Life of typical samples are given in TABLE I, with variations in preparation of the sample noted.
The data in TABLE I show that anodes of the present invention are effective for electrowinning nickel, and further that current densities of 30 mA/cm2 the anodes operate at very stable potentials in the neighborhood of about 1.19 to 1.4 volts/SCE.
EXAMPLE II
Th-s example illustrates the effect of various treatment conditions on the outer coating and on the inter-mediate layer of the composite anode of this invention.
A. Effect on Outer Layer Composite samples without barrier layers are prepared in a similar manner to that shown in EXAMPLE I, except that the final heat treatment in air of the RuC13-3H20 deposit is varied with respect to time and temperature. The samples are allowed to stand in lN H2SO4, at temperatures up to 70C. TABLE II-A shows the effect of variation in heat treatment of the Ru02 layer on the anode.
789~0 U~
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TABLE II-A
Treatment Temperature, C Time, min Effect 260 15-60Dissolution 315 30 Stable 370 30 Stable 425 15-60 Stable 455 30 Stable The results show that at a temperature-time cycle which does not convert the ruthenium chloride deposit to the oxide, the coating will dissolve immediately on contact with the acid. Coating adherence improves with higher heat treatment temperatures, at 455C, the adherence being demonstrably better than at 315 or 370C. The optimum time of heat treatment, as determined by tape tests, is about 30-60 minutes.
B Effect of Temperature-Time on Intermediate Layer Samples are prepared by plating a Ru-4~Ir alloy deposit on to a sandblasted, pumiced and cathodically cleaned titanium substrate. The ruthenium-iridium layer is subjected to various temperature-time cycles in air.
Thereafter the composites are tested as anodes in lN
H2SO4 as electrolyte, ambient temperature and at an anode current density of 5000 A/m2. TABLE II-B shows the effects of heat treatment conditions on the anode.
TABLE II-B
Heat Treatment Time in Hours to Cell Conditions Potential of 10 Volts - 426C- 1 hr -air - 3 30593C-15 min-air 150 593C-30 min-air 144 704C- 1 hr -air 36 11789~0 The results in TAsLE II show the preferred temperature-time cycle for heating the alloy is that equivalent to 593C for 15 to 30 minutes. At 704C for 1 hour the integrity of the co-deposit is damaged and the substrate is unduly oxidized.
At 426C for 1 hour insufficient oxide is formed.
C. Effect of Atmosphere on Allo~ Layer Samples are prepared in a similar manner to those prepared in part B of this example except that the atmo-sphere of the heat treatment of the ruthenium-4 weight ~
iridium alloy layer is varied. The composites are used as anodes in a simulated nickel electrowinning bath, substan-tially as described in EXAMPLE I, except that the bath is maintained at 55C. TABLE II-C gives a comparison of an electrode prepared by heat treating the alloy layer in an atmosphere of essentially pure 2 with one treated in air.
TABLE II-C
Time in Hours to Anode Heat Treatment Potential of 2 Volts 593C-15 min-O2 3200 593C-15 min-air 4200 EXAMPLE III
This example illustrates the effect of the ad-dition of titanium to the ruthenium oxide outer layer.
A composite is prepared in a similar manner to that shown in EXAMPLE I, except that titanium chloride in the amount of 15 weight %, based on the weight of titanium, is added to the RuCl3-3H20 solution, and the ruthenium coat-ing solution is made with methanol rather than butanol.
The ruthenium chloride solution used to deposit 9~0 the outer layer is prepared by dissolving RuC13.3H 2 and an aqueous solution of TiCl3 (20~) in methanol such that the ruthenium to titanium weight ratio is 85:15. The titanium is oxidized to the titanic (+4) state by the addition of H202. The resultant ruthenium- and titanium-containing solution is applied to the oxidized ruthenium-iridium alloy layer by applying several coats until the loading averages 1.2 mg/cm2. Each coat is allowed to dry under a heat lamp (65-93C) before the succeeding one is applied. After applying the final coat the electrode is heated in air for 30 minutes at 454C. The resultant material has a blue-black outer layer that has good adherence, showing onl slight coating lift-off in a tape test. Data for the tests are shown in TABLE III.
When tested in a simulated nickel electrowinning reco~ery cell, anodes of this type show an initial anodic potential substantially equivalent to that shown by coatings having a surface layer developed from a RuCl3-3H20 paint containing no TiC13. The life in TABLE III is shorter than the life for comparable electrodes without Tio2 in TABLE I.
Possibly the coating technique must be improved.
EXAMPLE IV
This example illustrates the effect of a palladium barrier layer and an ruthenium-iridium intermediate layer, in accordance with the present invention, as oxygen elec-trodes in various tests.
Composite samples are prepared on roughened and cleaned titanium with layers deposited essentially as de-scribed in EXAMPLE I, except that samples were prepared with _ 24 -117i~''32C~
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li78920 and without a palladium layer and with and without a ru-thenium-iridium layer. One sample was prepared with an electrodeposited ruthenium intermediate layer. Variations in composition, treatment of the layers and the manner of testing are noted.
Part A
In the tests recorded in TABLE IV-A, Samples 7 and 8 have a thin electroplated deposit of palladium of O.l~m thickness, heat treated at 593C for 1 hour in 5~ Hz/N2.
Samples 6, 7 and 8 have a surface coating of Ru02 formed from a ruthenium trichloride-containing paint deposit heat treated at 454C for 30 min. in air. The Ru02 loading is 0.5 mg/cm3. Sample 8 has an intermediate layer between the palladium layer and Ru02 layer of electrodeposited ruthenium-4% iridium. The ruthenium-iridium layer, which is 0.5~m in thickness is heated at 533C for 15 minutes in air before the outer Ru02 layer is applied. Samples 6, 7 and 8 are used as anodes in a simulated nickel electrowinning anolyte, as described in EXAMPLE I. Data showing the time vs. anode potential for oxygen evolution are shown in TABLE IV-A.
~17~39;2~
TABLE IV-A
Anode Potentials in Simulated Ni Electrowinning Cell Operated at 300 A/m and 70C
Time, Sample 6 Sample 7 Sample 8 hrs Ti/Ru02 Ti/Pd/RuO2 Ti/Pd/Ru-Ir/RuO2 1 1.28 1.20 1.20 100 1.27 1.23 1.27 336 2.7 1.25 1.26 500 -- 1.26 1.25 672 -- 1.28 1.25 1000 -- 1.30 1.26 1164 -- +2.0V 1.27 2000 -- -- 1.27 3000 ~~ -- 1.29 4000 -- -- 1.37 4200 -- -- >2.0V
The data in TABLE IV-A show: The electrode com-posed essentially of Ru02 on Ti ~Sample 6) operates at a good potential, but it has a short life as an oxygen electrode.
The electrodes having a Pd-barrier layer (Samples 7 and 8) have operating potentials comparable to the Ru02 working potential of Sample 6. The Ru-Ir intermediate layer increases the life of the oxygen electrode (Sample 8 vs.
Sample 7), the potentials for Sample 8 being stabilized and low for about 4000 hours, which is roughly 4 times the life of Sample 7 without the Ru-Ir layer. It will be appreciated that, within certain limits, an increase in RuO2 loading in the surface coating (i.e., the working layer) will increase li789~0 the life of the electrode. The limits in thickness of the coating will be dictated largely by the technique for ap-plying suitable Ru02 coatings of the desired thickness and by considerations of cost.
Part B
In the tests recorded in TABLE IV-B, Sample 9 is prepared in accordance with the present invention with a Pd-barrier layer, an electrodeposited Ru-4~Ir intermediate layer and an Ru02 surface layer. In Sample 10, the inter-mediate layer is electroplated Ru. Samples 9 and 10 are tests in a simulated nickel electrowinning anolyte essenti-ally the same as described in EXAMPLE I, but operated at 55 GC .
TABLE IV-B
Time in Hours to Anode Sample Anode Layers on Ti Potential of 2 Volts 9O.l~m Pd(l) 0.5~m Ru-Ir(2) >8407 0.5 mg/cm2 Ru02(3)(Still in Test) 10O.l~m Pd(1) 0.5~m Ru(2) 264 0.5 mg/cm2 Ru02(3) (1) Electroplated Deposit H/T = 593C - 1 hr - 5~ H2/N2 (2) Electroplated Deposit H/T = 593C - 15 min - air (3) Paint Deposit H/T = 454C - 30 min - air The data in Table IV-B show that the addition of iridium in the intermediate layer increases the life of the anode markedly.
1:1'789~0 Part C
In tests recorded in TABLE IV-C, Sample 11 which does not have a barrier layer is compared with Sample 12, in accordance with the present invention, as an oxygen elec-trode under severe conditions, viz. in lN H2S0~ electrolvte at 5000 A/m2.
TABLE IV-C
Time in Hours to Cell Sample Anode Lavers on Ti Potential of 10 ~701ts 110.5~m Ru-Ir(2) 1.1 mg/cm2 Ru02(3) 110 120.2~m Pd(l) 0.5~m Ru-Ir(2) 250 1.1 mg/cm2 Ru02(3) (l)Electroplated deposit (no H/T) (2)Electroplated deposit H/T = 593C-15 min-air (3)Paint deposit H/T = 454C-30 min-air The data in TABLE IV-C shows that the palladium barrier layer increases the dura~ility of the anode.
EXAMPLE V
This example illustrates variations in the barrier layer.
Composite samples are prepared with a variety of metals electroplated on roughened and cleaned titanium sheet, followed by an electroplated layer of Ru-4~Ir. Data showing the results of tests using such composites as anodes in a simulated nickel electrowinning electrolyte, essen- _ tially as described in EXAMPLE I, are given in TABLE V. The thickness of the various deposits and treatments to which the deposits are subjected (if any) are noted. , :117892~
TABLE V
Anode Time in Hours to SampleLayers on TiAnode Potential of 2 Volts V-lO.l~m Pd(l) >8000 l.O~m Ru-Ir(2) (Still in Test) V-2O.l~m Pt(3) 2230 l.l~m Ru-Ir(2) V-3O.l~m Pt(4) 4510 l.O~m Ru-Ir(2) V-40.07 mg/cm2 Ir (x)>7410 l.O~m Ru-Ir(2) (Still in Test) V-5 None 2136 l.O~m Ru-Ir(2) V-6Flash Coating Au(x)(>213)*
l.l~m Ru-Ir(2) V-7 None ( 114)*
l.O~m Ru-Ir(2) Conditioning treatments:
(1)593C- 1 hr -5~ H2/N2 (2)593C-15 min-air (3)593C- 1 hr -N2 (4)593C- 1 hr -5~ H2/N2 + 72 hours room temperature (x)No Treatment (*)Under accelerated test in 1 N H2SO~ at current density of 500 mA/cm2 and ambient temperature to 10 volts cell voltage The data shows that Ir and Pd are particularly suitable as barrier layers and that an oxidation treatment improved the effectiveness of the platinum barrier layer.
It is noted that the Pd layer in Sample V-l was treated in a reducing atmosphere; as noted previously this treatment is not necessary for an effective Pd barrier layer. However, platinum requires the treatment in an oxidizing medium to be effective. Such platinum treatment is preferably carried out at room temperature.
EXA~PLE VI
This example shows the effect of variations in thickness of the Ru-Ir and Pd layers.
Part A - Variations in Thickness of ~u-Ir Composite tri-layer samples, viz. Pd/Ru-Ir/
Ru02 on Ti, in accordance with the present invention, are prepared essentially the same as described in EXA~PLE I, with variations in thickness in the Ru-Ir layer. In the samples prepared the Pd and Ru02 are constant, viz.
Pd = O.l~m, H/T = 593C - 1 hr -5~ H2/N2 or no treatment Ru02 = 0.5 mg/cm 2, H/T 454C -30 min in air.
The data in TABLE VI records the hours to 2V when tested in the simulated nickel electrowinning anolyte using the con-ditions noted in EXAMPLE I.
TABLE VI
Time in Hours to Sample Intermediate Layer Anode Potential of 2 Volts 13Ru-4%Ir = 0.5~m, H/T-593C-15 min-air 4200 14Ru-4%Ir = l~m, H/T-593C-15 min~air >9330 (Still in Test) 15Ru-4~Ir = 2~m, H/T-593C-15 min-air >9640 (Still in Test) 16Ru-4%Ir = 4~m, H/T-593C-15 min-air 2500 The data in TABLE VI show that electrodes of the present invention operate effectively with the variation in thickness of the Ru-4~Ir coating of from 0.5-4~m, and the optimum thickness is in the range of about 1-3~m.
li'7~39;~
Part B - Variarion in hickness of Pd Sam21es are prepared of electroplated palladium on roughened and cleaned titanium sheet, with the thickness o~
the Pd-deposit varying from about 0.05 to about lum, l.e., up to about 1.3 mg/cm2 Pd. The samples are tested as oxygen electrodes in lN H2SO~ at room temperature. A gra?h of potentials of the electrodes when operating at a constant current density of 2 mA/cm2 as a function of Pd-loading shows that at a Pd level greater than 0.2 mg/cm~, the surface behaves like pure Pd, an indication that the ti-tanium surface is completely covered with palladium. Belo~
about 0.2 mg/cm2 of palladium, the titanium substrate in-fluences the potential, as evidenced by the rise in po-tential as the Pd loading decreases belo~ about 0.2 mg/cr .
EXA~IPLE VII
This example illustrates the effect of iridium, the effect of an oxidation treatment in the intermediate layer, and the contribution of the RuO2 layers of the present invention in tests as oxygen electrodes.
Composite samples are prepared, all having an electroplated ruthenium-containing layer with an iridium content varied from 0 up to about 12%. The electroplated layer is deposited directly on roughened and cleaned ti-tanium. Each sample has an electrodeposit of about 1 mg/cm2 loading~ Thereafter, with the exception of Samples _ 24 and 25, each sample is subjected to a treatment at 593C
in air for 15 minutes. Samples 18, 20 and 24 each have a further outer layer of RuO2 (0.8 mg/cm2) developed from a ~789~
ruthenium chloride-containing paint, which is subjected to a heat treatment of 450C for 30 hours in air. Sample 25 is comparable to Sample 21, except that it does not have an oxidation treatment. The samples are used as anodes in a lN
H2SO4 electrolyte operated at incremental current densities until a color change in the electrolyte is observed. White Teflon (Teflon is a DuPont Trademark) tape inserted at the stopper for each test is removed and examined. Effluent gas from the test container is bubbled through a solution of 1:5 of H2SO3:H2O. No noticeable change occurs in the H2SO3.
Observations are reported in TABLE VII.
The results in TABLE VII show:
1) The presence of Ir suppresses the corrosion of Ru.
As the iridium content increases from 0 to 3.9 to 9.4% the current density at which coloring of the electrolyte begins rises from 30 to 250 mA/cm2, and the deposits of RuO2 2H2O on the tape decrease from black amounts to trace amounts (Cf Samples 17, 19, 22)~
2) The presence of RuO2 arising on the surface as the result of a non-electroplated deposit suppresses the formation of a ruthenium-containing deposit on the tape, believed to be RuO2-2H2O (via Ru04 formation), and the corrosion of Ru in all cases. With no Ir present, there is less of such a deposit on the tape with a non-electroplated RuO2 surface layer than without it (Cf Samples 17 and 18), and corrosion begins at a higher current density. In Ru-Ir deposits not heat treated, the ruthenium-containing deposits on the tape are lesser when RuO2 is present, and the corrosion of Ru occurs also to a lesser degree (Cf Samples 25 and 24). When the Ru-Ir deposit is heat 9~
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and RuO2 is present, no ruthenium-containing deposit on the tape is found at current densities up to about 250 mA/cm2 (Cf Sample 20).
3) Further the results show oxidation of the Ru-Ir layer is necessary to form a proective oxide film. When Ru-Ir was not heat treated, corrosion of Ru began at 30 mA/cm2 and a black-brown deposit was present on the tape. When Ru-Ir was heat-treated, corrosion began at much higher current densities, and the volatiles were reduced to trace amounts (Cf Samples 21 and 25).
From the results it can be seen that the optimum amount of iridium in the Ru-Ir can be predetermined for given conditions of operation based upon, e.g., corrosion and economics. For example, the Sample 20 containing about 3.9%
iridium and having an RuO2 outer coating may be used at current densities up to 250 mA/cm2 without noticeable dissolution of the ruthenium in the electrolyte. It appears from the data that less than 4~ iridium may be used with the RuO2 for lower current densities of the order of 30-50 mA/cm2, e.g. 1% or 2%
may be sufficient.
This example illustrates the effect of the iridium level in a ruthenium-iridium layer.
In the experiments of this exam~le composite samples composed of a ruthenium-iridium electroplated deposit on roughened and cleaned titanium are tested in an accelerated life test~ The ruthenium-iridium deposits contain various amounts from zero up to about 25% iridium (by weight).
11';~89~0 Results with typical samples prepared under comparable conditions are reported in TABLE VIII.
TABLE VIII
Time in Hours to Cell Sample _ Potential of 10 Volts 26 0 0.3 27 0.7 95
predominant phase present is Ru02, which may be in solid so-lution with other oxides which develop at the surface.
In view of the dependence on the conditions of use, the electrode can be designed with the appropriate amount of iridium. For reasons of cost, consistent with electrode life, it is preferable to keep the iridium level as low as possible.
The surface layer in a preferred anode of this invention contains as an essential component ruthenium dioxide which has been developed from a non-electrolytically deposited source. This, as noted above, is to ensure that even initially there is no loss of ruthenium anodically in use. Ruthenium dioxide is known to have a low oxygen over-potential, and its presence at the surface as an additional layer will also optimize the effectiveness of the material as an oxygen electrode. This in turn will enable the use of li7892~
the electrode at a sufficiently low potential to minimize the possibility of initial dissolution of ruthenium. Other non-electrolytically active components may be present, e.g.
for adherence, e.g., an oxide of substrate components such as TiO2, Ta2O5 and the like. In a preferred embodiment of the invention the outer surface layer contains at least abou-t 8090 RuO2. In the embodiment in which a non-active component is present the outer surface layer contains about 80% to about 99% ruthenium dioxide and about 1% to about 20%
of the non-active component, e.g., titanium dioxide. Suit-able outer layers may contain ~or example, 80% RuO2-20~
TiO2, 85% RuO2-15~ TiO2, 90% RuO2-10% TiO2, 80% RuO2-10%
TiO2-10~o Ta2O5. It is believed, however, that the require-ment for a non-active component such as a valve metal oxide is less critical and may even be eliminated in the present electrodes. The reason for this is -that the thickness requirements of the outer (non-electrolytic) RuO2 deposit are not as critical in the present electrodes as in conventional electrodes made entirely of a paint-type deposit. Con-ventional paint-type electrodes require a thickness build-up in sequential deposits that have been reported to be as high as 8 coatings and higher with firing steps intermit-tently in the build-up. Since the RuO2 (non-electrolytically deposited) layer can be thir,ner in the present electrodes, with no more than, for example, 1 or 2 coatings, the re-quirement for additional binders is lowered. Indeed durable anodes have been made using as the outer surface layer and a Ru-Ir layer, a RuO2 developed from paints without any ad-ditional oxide component. Where resinates, or the li~e are .
:~ 1'7~9;~0 used, some oxides may be derived from the usual commercial formulations, but such paint formulations can be applied without any additional oxides added.
Any non-electrolytic technique can be used for prGducing the ruthenium dioxide containing outer surface layer. Many methods are known, ~or example, for developing ruthenium dioxide coatings from aqueous or organic vehicles containing ruthenium values. For example, the ruthenium may be present as a compound such as a halide or resinate, which oxidizes to ruthenium dioxide when subjected to a heat treatment in an oxidizing atmosphere. Several methods for developing ruthenium dioxide surface coatings from non-electroplated coatings are described in the patents cited previously. In one method a ruthenium chloride in solution is applied as a paint and the coating of ruthenium dioxide is formed by dechlorination and oxidation of the ruthenium chloride. For example, a solution of RuCl3.3H20 in a suit-able carrier may be applied on a previously coated and treated composite by brushing, spraying or dipping. A
sufficient number of coats are applied to provide a ru-thenium content of at least about 0.1 mg/cm2 of electrode surface area. The coatings may be fired individually or each may be allowed to dry and the final coating fired.
Firing is carried out, e.g., in air at a temperature of about 315C to about 455C, e.g., about 315C to about 455C
for about 15 to about 60 minutes. Titanium or other non-active components may be co-deposited with the ruthenium using conventional techniques. Typically the initial loading (i.e. prior to build-up in use) of the RuO2-containing outer layer is at least about 0.1 mg/cm2. Preferably, the initial loading is about 0.3 to about 1 mg/cm2 in thickness. Since there is usually a build-up of RuO2 during use in the cell, the initial thickness of RuO2 is to ensure that preci~us metals of the intermediate layer do not dissolve before the proper build-up of RuO2 can occur and to ensure a low oxygen overpotential in the cell. In this way precious ~etal loss is minimized.
As indicated above, in a preferred embodiment of the invention the composite electrode is used as an insoluble anode for the electrowinning of nickel. While it is not the intention to confine the use of the electrodes to any one process, one contemplated use of the present electrode is in nickel electrowinning processes. For example, nickel electrowinning processes are known which use electrolytes containing about 40 to lO0 g/l nickel, 50 to lO0 g/l sodium sulfate and ùp to 40 g/l boric acid in sulfuric acid to maintain a pH ~n the range of about 0 to 5.5. In one such electrowinning process the anode is bagged, and the anolyte is a sulfate solution containing about 40 to 70 g/l nickel (as nickel sulfate), 40 g/l sulfuric acid, 100 g/l sodium sulfate, 40 g/l boric acid, and the anolyte is at a pH of about 0.
Electrowinning is carried out advantageously at a temperature of about 50 to 70C and at an anode current denslty of about 30-50 milliamps per square centimeter (mA/cm2).
The following examples are intended to give those skilled in the art a better appreciation of the invention. In all the tests, anode potentials are measured in volts vs. a saturated calomel electrode (SCE) and H/T is an abbreviation 11'~89ZO
to denote the conditioning of the layer of a composite sample, viz. the temperature, time and atmosphere. Load-ings, e.g. of precious metals or their oxides, alloys, etc., in various layers are given as nominal values.
EXAMPLE I
This example illustrates the preparation of typical electrodes of the present invention, in which the barrier layer is palladium, and the activity of such electrodes when used as anodes for the electrowinning of nickel.
Several multilayer samples are prepared on a titanium substrate material as follows:
Surface roughened titanium sheet is cleaned and plated with a thin coating of a precious metal as a barrier layer.
To roughen and clean the titanium it is sandblasted with SiOz-sand, brushed with pumice, rinsed, cathodically cleaned in 0.5 M Na2CO3 to remove dirt and the remaining pumice particles then rinsed and dried. Thereafter, the cleaned substrate is plated with a thin deposit of palladium, the amount varying from about 0.1 to about 0.6~m, using known electroplàting baths. In some of the samples the palladium deposit i5 subjected to special treatment. For example, the palladium coated-titanium in some samples are subjected to a temperature of 593C for 1 hour in an atmosphere of 5% ~2-Bal N2. It was found during the course of investigating the materials that such treatment of the palladium layer could be eliminated without noticeable harmful effects in the electrode life or performance.
A ruthenium-iridium intermediate, e.g., of about .
1/2 to about 4~m thickness, is plated on the palladium layer from a sulfamate bath to give a deposit containing about 4~
iridium and the balance ruthenium. The bath, which is dis-closed in the co-pending application referred to above, is maintained at a pH of 0.9 and a temperature of 57C and operated at a current density of 20 mA/cm2. The ruthenium-iridium deposit is treated in air at a temperature of about 500 to 600C for about 10 to 20 minutes to oxidize the surface.
The surface Ru02 layer is applied to each sample by painting the composite with 2 coats of a solution of RuCl3-3H20 in n-butanol. After each application the elec-trode is dried under a heat lamp (about 65-93C) to obtain a ru~henium chloride loading of about 1 mg/cm2, and then the composite is heat treated in air for 60 minutes at about 450C to about 600C in order to convert the chloride to the dioxide of ruthenium.
A uniform, blue-black coating results which is adherent when finger rubbed, but not completely adherent when subjected to a tape test. The tape test involves firmly applying a strip of tape to the coating and rapidly stripping the tape off. The tape is then examined to see whether any of the coating has been pulled off from the substrate.
The samples are tested as anodes under conditions which simulate the anolyte in a bagged-anode nickel electro-winning, viz. an aqueous electrolyte composed of 70 g/l nickel (as nickel sulfate), 40 g/l sulfuric acid, 100 g/l ~17892(~
sodium sulfate, and 10 g/l boric acid. The bath is main-tained at a temperature of 70C, a pH of O to 0.5, and an anode current density of 30 mA/cm2. The tests are arbi-trarily terminated when the anode potential reaches 2 volts (vs. SCE).
Life of typical samples are given in TABLE I, with variations in preparation of the sample noted.
The data in TABLE I show that anodes of the present invention are effective for electrowinning nickel, and further that current densities of 30 mA/cm2 the anodes operate at very stable potentials in the neighborhood of about 1.19 to 1.4 volts/SCE.
EXAMPLE II
Th-s example illustrates the effect of various treatment conditions on the outer coating and on the inter-mediate layer of the composite anode of this invention.
A. Effect on Outer Layer Composite samples without barrier layers are prepared in a similar manner to that shown in EXAMPLE I, except that the final heat treatment in air of the RuC13-3H20 deposit is varied with respect to time and temperature. The samples are allowed to stand in lN H2SO4, at temperatures up to 70C. TABLE II-A shows the effect of variation in heat treatment of the Ru02 layer on the anode.
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Treatment Temperature, C Time, min Effect 260 15-60Dissolution 315 30 Stable 370 30 Stable 425 15-60 Stable 455 30 Stable The results show that at a temperature-time cycle which does not convert the ruthenium chloride deposit to the oxide, the coating will dissolve immediately on contact with the acid. Coating adherence improves with higher heat treatment temperatures, at 455C, the adherence being demonstrably better than at 315 or 370C. The optimum time of heat treatment, as determined by tape tests, is about 30-60 minutes.
B Effect of Temperature-Time on Intermediate Layer Samples are prepared by plating a Ru-4~Ir alloy deposit on to a sandblasted, pumiced and cathodically cleaned titanium substrate. The ruthenium-iridium layer is subjected to various temperature-time cycles in air.
Thereafter the composites are tested as anodes in lN
H2SO4 as electrolyte, ambient temperature and at an anode current density of 5000 A/m2. TABLE II-B shows the effects of heat treatment conditions on the anode.
TABLE II-B
Heat Treatment Time in Hours to Cell Conditions Potential of 10 Volts - 426C- 1 hr -air - 3 30593C-15 min-air 150 593C-30 min-air 144 704C- 1 hr -air 36 11789~0 The results in TAsLE II show the preferred temperature-time cycle for heating the alloy is that equivalent to 593C for 15 to 30 minutes. At 704C for 1 hour the integrity of the co-deposit is damaged and the substrate is unduly oxidized.
At 426C for 1 hour insufficient oxide is formed.
C. Effect of Atmosphere on Allo~ Layer Samples are prepared in a similar manner to those prepared in part B of this example except that the atmo-sphere of the heat treatment of the ruthenium-4 weight ~
iridium alloy layer is varied. The composites are used as anodes in a simulated nickel electrowinning bath, substan-tially as described in EXAMPLE I, except that the bath is maintained at 55C. TABLE II-C gives a comparison of an electrode prepared by heat treating the alloy layer in an atmosphere of essentially pure 2 with one treated in air.
TABLE II-C
Time in Hours to Anode Heat Treatment Potential of 2 Volts 593C-15 min-O2 3200 593C-15 min-air 4200 EXAMPLE III
This example illustrates the effect of the ad-dition of titanium to the ruthenium oxide outer layer.
A composite is prepared in a similar manner to that shown in EXAMPLE I, except that titanium chloride in the amount of 15 weight %, based on the weight of titanium, is added to the RuCl3-3H20 solution, and the ruthenium coat-ing solution is made with methanol rather than butanol.
The ruthenium chloride solution used to deposit 9~0 the outer layer is prepared by dissolving RuC13.3H 2 and an aqueous solution of TiCl3 (20~) in methanol such that the ruthenium to titanium weight ratio is 85:15. The titanium is oxidized to the titanic (+4) state by the addition of H202. The resultant ruthenium- and titanium-containing solution is applied to the oxidized ruthenium-iridium alloy layer by applying several coats until the loading averages 1.2 mg/cm2. Each coat is allowed to dry under a heat lamp (65-93C) before the succeeding one is applied. After applying the final coat the electrode is heated in air for 30 minutes at 454C. The resultant material has a blue-black outer layer that has good adherence, showing onl slight coating lift-off in a tape test. Data for the tests are shown in TABLE III.
When tested in a simulated nickel electrowinning reco~ery cell, anodes of this type show an initial anodic potential substantially equivalent to that shown by coatings having a surface layer developed from a RuCl3-3H20 paint containing no TiC13. The life in TABLE III is shorter than the life for comparable electrodes without Tio2 in TABLE I.
Possibly the coating technique must be improved.
EXAMPLE IV
This example illustrates the effect of a palladium barrier layer and an ruthenium-iridium intermediate layer, in accordance with the present invention, as oxygen elec-trodes in various tests.
Composite samples are prepared on roughened and cleaned titanium with layers deposited essentially as de-scribed in EXAMPLE I, except that samples were prepared with _ 24 -117i~''32C~
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li78920 and without a palladium layer and with and without a ru-thenium-iridium layer. One sample was prepared with an electrodeposited ruthenium intermediate layer. Variations in composition, treatment of the layers and the manner of testing are noted.
Part A
In the tests recorded in TABLE IV-A, Samples 7 and 8 have a thin electroplated deposit of palladium of O.l~m thickness, heat treated at 593C for 1 hour in 5~ Hz/N2.
Samples 6, 7 and 8 have a surface coating of Ru02 formed from a ruthenium trichloride-containing paint deposit heat treated at 454C for 30 min. in air. The Ru02 loading is 0.5 mg/cm3. Sample 8 has an intermediate layer between the palladium layer and Ru02 layer of electrodeposited ruthenium-4% iridium. The ruthenium-iridium layer, which is 0.5~m in thickness is heated at 533C for 15 minutes in air before the outer Ru02 layer is applied. Samples 6, 7 and 8 are used as anodes in a simulated nickel electrowinning anolyte, as described in EXAMPLE I. Data showing the time vs. anode potential for oxygen evolution are shown in TABLE IV-A.
~17~39;2~
TABLE IV-A
Anode Potentials in Simulated Ni Electrowinning Cell Operated at 300 A/m and 70C
Time, Sample 6 Sample 7 Sample 8 hrs Ti/Ru02 Ti/Pd/RuO2 Ti/Pd/Ru-Ir/RuO2 1 1.28 1.20 1.20 100 1.27 1.23 1.27 336 2.7 1.25 1.26 500 -- 1.26 1.25 672 -- 1.28 1.25 1000 -- 1.30 1.26 1164 -- +2.0V 1.27 2000 -- -- 1.27 3000 ~~ -- 1.29 4000 -- -- 1.37 4200 -- -- >2.0V
The data in TABLE IV-A show: The electrode com-posed essentially of Ru02 on Ti ~Sample 6) operates at a good potential, but it has a short life as an oxygen electrode.
The electrodes having a Pd-barrier layer (Samples 7 and 8) have operating potentials comparable to the Ru02 working potential of Sample 6. The Ru-Ir intermediate layer increases the life of the oxygen electrode (Sample 8 vs.
Sample 7), the potentials for Sample 8 being stabilized and low for about 4000 hours, which is roughly 4 times the life of Sample 7 without the Ru-Ir layer. It will be appreciated that, within certain limits, an increase in RuO2 loading in the surface coating (i.e., the working layer) will increase li789~0 the life of the electrode. The limits in thickness of the coating will be dictated largely by the technique for ap-plying suitable Ru02 coatings of the desired thickness and by considerations of cost.
Part B
In the tests recorded in TABLE IV-B, Sample 9 is prepared in accordance with the present invention with a Pd-barrier layer, an electrodeposited Ru-4~Ir intermediate layer and an Ru02 surface layer. In Sample 10, the inter-mediate layer is electroplated Ru. Samples 9 and 10 are tests in a simulated nickel electrowinning anolyte essenti-ally the same as described in EXAMPLE I, but operated at 55 GC .
TABLE IV-B
Time in Hours to Anode Sample Anode Layers on Ti Potential of 2 Volts 9O.l~m Pd(l) 0.5~m Ru-Ir(2) >8407 0.5 mg/cm2 Ru02(3)(Still in Test) 10O.l~m Pd(1) 0.5~m Ru(2) 264 0.5 mg/cm2 Ru02(3) (1) Electroplated Deposit H/T = 593C - 1 hr - 5~ H2/N2 (2) Electroplated Deposit H/T = 593C - 15 min - air (3) Paint Deposit H/T = 454C - 30 min - air The data in Table IV-B show that the addition of iridium in the intermediate layer increases the life of the anode markedly.
1:1'789~0 Part C
In tests recorded in TABLE IV-C, Sample 11 which does not have a barrier layer is compared with Sample 12, in accordance with the present invention, as an oxygen elec-trode under severe conditions, viz. in lN H2S0~ electrolvte at 5000 A/m2.
TABLE IV-C
Time in Hours to Cell Sample Anode Lavers on Ti Potential of 10 ~701ts 110.5~m Ru-Ir(2) 1.1 mg/cm2 Ru02(3) 110 120.2~m Pd(l) 0.5~m Ru-Ir(2) 250 1.1 mg/cm2 Ru02(3) (l)Electroplated deposit (no H/T) (2)Electroplated deposit H/T = 593C-15 min-air (3)Paint deposit H/T = 454C-30 min-air The data in TABLE IV-C shows that the palladium barrier layer increases the dura~ility of the anode.
EXAMPLE V
This example illustrates variations in the barrier layer.
Composite samples are prepared with a variety of metals electroplated on roughened and cleaned titanium sheet, followed by an electroplated layer of Ru-4~Ir. Data showing the results of tests using such composites as anodes in a simulated nickel electrowinning electrolyte, essen- _ tially as described in EXAMPLE I, are given in TABLE V. The thickness of the various deposits and treatments to which the deposits are subjected (if any) are noted. , :117892~
TABLE V
Anode Time in Hours to SampleLayers on TiAnode Potential of 2 Volts V-lO.l~m Pd(l) >8000 l.O~m Ru-Ir(2) (Still in Test) V-2O.l~m Pt(3) 2230 l.l~m Ru-Ir(2) V-3O.l~m Pt(4) 4510 l.O~m Ru-Ir(2) V-40.07 mg/cm2 Ir (x)>7410 l.O~m Ru-Ir(2) (Still in Test) V-5 None 2136 l.O~m Ru-Ir(2) V-6Flash Coating Au(x)(>213)*
l.l~m Ru-Ir(2) V-7 None ( 114)*
l.O~m Ru-Ir(2) Conditioning treatments:
(1)593C- 1 hr -5~ H2/N2 (2)593C-15 min-air (3)593C- 1 hr -N2 (4)593C- 1 hr -5~ H2/N2 + 72 hours room temperature (x)No Treatment (*)Under accelerated test in 1 N H2SO~ at current density of 500 mA/cm2 and ambient temperature to 10 volts cell voltage The data shows that Ir and Pd are particularly suitable as barrier layers and that an oxidation treatment improved the effectiveness of the platinum barrier layer.
It is noted that the Pd layer in Sample V-l was treated in a reducing atmosphere; as noted previously this treatment is not necessary for an effective Pd barrier layer. However, platinum requires the treatment in an oxidizing medium to be effective. Such platinum treatment is preferably carried out at room temperature.
EXA~PLE VI
This example shows the effect of variations in thickness of the Ru-Ir and Pd layers.
Part A - Variations in Thickness of ~u-Ir Composite tri-layer samples, viz. Pd/Ru-Ir/
Ru02 on Ti, in accordance with the present invention, are prepared essentially the same as described in EXA~PLE I, with variations in thickness in the Ru-Ir layer. In the samples prepared the Pd and Ru02 are constant, viz.
Pd = O.l~m, H/T = 593C - 1 hr -5~ H2/N2 or no treatment Ru02 = 0.5 mg/cm 2, H/T 454C -30 min in air.
The data in TABLE VI records the hours to 2V when tested in the simulated nickel electrowinning anolyte using the con-ditions noted in EXAMPLE I.
TABLE VI
Time in Hours to Sample Intermediate Layer Anode Potential of 2 Volts 13Ru-4%Ir = 0.5~m, H/T-593C-15 min-air 4200 14Ru-4%Ir = l~m, H/T-593C-15 min~air >9330 (Still in Test) 15Ru-4~Ir = 2~m, H/T-593C-15 min-air >9640 (Still in Test) 16Ru-4%Ir = 4~m, H/T-593C-15 min-air 2500 The data in TABLE VI show that electrodes of the present invention operate effectively with the variation in thickness of the Ru-4~Ir coating of from 0.5-4~m, and the optimum thickness is in the range of about 1-3~m.
li'7~39;~
Part B - Variarion in hickness of Pd Sam21es are prepared of electroplated palladium on roughened and cleaned titanium sheet, with the thickness o~
the Pd-deposit varying from about 0.05 to about lum, l.e., up to about 1.3 mg/cm2 Pd. The samples are tested as oxygen electrodes in lN H2SO~ at room temperature. A gra?h of potentials of the electrodes when operating at a constant current density of 2 mA/cm2 as a function of Pd-loading shows that at a Pd level greater than 0.2 mg/cm~, the surface behaves like pure Pd, an indication that the ti-tanium surface is completely covered with palladium. Belo~
about 0.2 mg/cm2 of palladium, the titanium substrate in-fluences the potential, as evidenced by the rise in po-tential as the Pd loading decreases belo~ about 0.2 mg/cr .
EXA~IPLE VII
This example illustrates the effect of iridium, the effect of an oxidation treatment in the intermediate layer, and the contribution of the RuO2 layers of the present invention in tests as oxygen electrodes.
Composite samples are prepared, all having an electroplated ruthenium-containing layer with an iridium content varied from 0 up to about 12%. The electroplated layer is deposited directly on roughened and cleaned ti-tanium. Each sample has an electrodeposit of about 1 mg/cm2 loading~ Thereafter, with the exception of Samples _ 24 and 25, each sample is subjected to a treatment at 593C
in air for 15 minutes. Samples 18, 20 and 24 each have a further outer layer of RuO2 (0.8 mg/cm2) developed from a ~789~
ruthenium chloride-containing paint, which is subjected to a heat treatment of 450C for 30 hours in air. Sample 25 is comparable to Sample 21, except that it does not have an oxidation treatment. The samples are used as anodes in a lN
H2SO4 electrolyte operated at incremental current densities until a color change in the electrolyte is observed. White Teflon (Teflon is a DuPont Trademark) tape inserted at the stopper for each test is removed and examined. Effluent gas from the test container is bubbled through a solution of 1:5 of H2SO3:H2O. No noticeable change occurs in the H2SO3.
Observations are reported in TABLE VII.
The results in TABLE VII show:
1) The presence of Ir suppresses the corrosion of Ru.
As the iridium content increases from 0 to 3.9 to 9.4% the current density at which coloring of the electrolyte begins rises from 30 to 250 mA/cm2, and the deposits of RuO2 2H2O on the tape decrease from black amounts to trace amounts (Cf Samples 17, 19, 22)~
2) The presence of RuO2 arising on the surface as the result of a non-electroplated deposit suppresses the formation of a ruthenium-containing deposit on the tape, believed to be RuO2-2H2O (via Ru04 formation), and the corrosion of Ru in all cases. With no Ir present, there is less of such a deposit on the tape with a non-electroplated RuO2 surface layer than without it (Cf Samples 17 and 18), and corrosion begins at a higher current density. In Ru-Ir deposits not heat treated, the ruthenium-containing deposits on the tape are lesser when RuO2 is present, and the corrosion of Ru occurs also to a lesser degree (Cf Samples 25 and 24). When the Ru-Ir deposit is heat 9~
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and RuO2 is present, no ruthenium-containing deposit on the tape is found at current densities up to about 250 mA/cm2 (Cf Sample 20).
3) Further the results show oxidation of the Ru-Ir layer is necessary to form a proective oxide film. When Ru-Ir was not heat treated, corrosion of Ru began at 30 mA/cm2 and a black-brown deposit was present on the tape. When Ru-Ir was heat-treated, corrosion began at much higher current densities, and the volatiles were reduced to trace amounts (Cf Samples 21 and 25).
From the results it can be seen that the optimum amount of iridium in the Ru-Ir can be predetermined for given conditions of operation based upon, e.g., corrosion and economics. For example, the Sample 20 containing about 3.9%
iridium and having an RuO2 outer coating may be used at current densities up to 250 mA/cm2 without noticeable dissolution of the ruthenium in the electrolyte. It appears from the data that less than 4~ iridium may be used with the RuO2 for lower current densities of the order of 30-50 mA/cm2, e.g. 1% or 2%
may be sufficient.
This example illustrates the effect of the iridium level in a ruthenium-iridium layer.
In the experiments of this exam~le composite samples composed of a ruthenium-iridium electroplated deposit on roughened and cleaned titanium are tested in an accelerated life test~ The ruthenium-iridium deposits contain various amounts from zero up to about 25% iridium (by weight).
11';~89~0 Results with typical samples prepared under comparable conditions are reported in TABLE VIII.
TABLE VIII
Time in Hours to Cell Sample _ Potential of 10 Volts 26 0 0.3 27 0.7 95
6.1 114 31 6.3 120 32 8.1 112 33 9.4 118 21.3 426 It will be appreciated that the selected results reported in TABLE VIII are for rough screening tests. Some tests not reported in the table showed poor performance at high levels of iridium and good life at low levels of iridium. However, the life of the electrodes will vary markedly depending on such factors as the type of bath used, plating conditions, thickness of the coating, treatment conditions, integrity of the deposit, etc. It is believed, however, that the results tabulated in TABLE VIII are for relatively comparable samples and that in general the ex-periments showed a trend, as indicated.
As noted previously the present anodes are par-ticularly useful for electrowinning nickel. The electrodes l:~';'B~
may also be used for recovering nickel-cobalt deposits from a suitable electrolyte under comparable conditions and with suitably low anode potentials, e.g. of the order of about 1.15-1.3V/SCE.
Although the present invention has been describec in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art will readily under-stand. Such modifications and variations are considered to be within the purview and scope of the invention and ap-pended claims.
~ 37 ~ L
As noted previously the present anodes are par-ticularly useful for electrowinning nickel. The electrodes l:~';'B~
may also be used for recovering nickel-cobalt deposits from a suitable electrolyte under comparable conditions and with suitably low anode potentials, e.g. of the order of about 1.15-1.3V/SCE.
Although the present invention has been describec in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art will readily under-stand. Such modifications and variations are considered to be within the purview and scope of the invention and ap-pended claims.
~ 37 ~ L
Claims (8)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. In a process for producing a composite electrode for use in an electrolytic cell comprising a valve metal substrate and an outer surface layer comprising ruthenium dioxide, the improvement which comprises provid-ing a barrier layer comprising a platinum group metal directly on the substrate, and providing, between the barrier layer and outer surface layer, an intermediate layer comprising a metallic electroplated deposit consisting of ruthenium and iridium, said intermediate layer containing at least a small but effective amount of iridium to reduce ruthenium dissolution during use in said cell, said intermediate layer being at least partially oxidized.
2. A process according to claim 1, wherein the intermediate layer deposited between the barrier layer and outer surface layer is subjected to a heat treatment in an oxidizing atmosphere to oxidize at least a portion of the outer surface of said layer.
3. A process according to claim 2, wherein the heat treatment is effected at a temperature of about 400°C to about 900°C in an oxidizing atmosphere.
4. A process according to claim 1, wherein the metallic electroplated layer of ruthenium and iridium is subjected to a temperature of about 400°C
to about 900°C for about 5 to about 60 minutes in an oxidizing atmosphere to at least partially oxidize the surface of said layer before depositing the outer surface layer.
to about 900°C for about 5 to about 60 minutes in an oxidizing atmosphere to at least partially oxidize the surface of said layer before depositing the outer surface layer.
5. A process according to claim 1, wherein the platinum group metal of the barrier layer is provided directly on the substrate by means of electroplating.
6. A process according to claim 1, wherein the platinum group metal of the barrier layer is platinum and the barrier layer is treated in an oxidizing medium prior to deposition of the intermediate layer.
7. A process according to claim 1, wherein the ruthenium dioxide layer is developed by decomposition and oxidation of a ruthenium compound deposited in a vehicle on the intermediate layer.
8. A process according to claim 1, wherein the ruthenium dioxide outer layer is developed at a temperature of 315°C to 455°C in an oxidizing atmosphere.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000430587A CA1178920A (en) | 1978-07-14 | 1983-06-16 | Composite electrode for electrolytic processes |
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US924,631 | 1978-07-14 | ||
| US05/924,631 US4157943A (en) | 1978-07-14 | 1978-07-14 | Composite electrode for electrolytic processes |
| CA000330104A CA1153731A (en) | 1978-07-14 | 1979-06-19 | Composite electrode for electrolytic processes |
| CA000430587A CA1178920A (en) | 1978-07-14 | 1983-06-16 | Composite electrode for electrolytic processes |
Related Parent Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000330104A Division CA1153731A (en) | 1978-07-14 | 1979-06-19 | Composite electrode for electrolytic processes |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1178920A true CA1178920A (en) | 1984-12-04 |
Family
ID=27166297
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000430587A Expired CA1178920A (en) | 1978-07-14 | 1983-06-16 | Composite electrode for electrolytic processes |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1178920A (en) |
-
1983
- 1983-06-16 CA CA000430587A patent/CA1178920A/en not_active Expired
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